Why do guys act jealous when you're not even in a relationship?

Quick rundown: Had a friends with benefits situation with a guy, told him that I was talking to another guy and couldn't sleep with him anymore and he seemed sort of nonchalant about it. We didn't talk for a week and tonight he messages me asking things like "How are you and the new guy?" "Are you happy?" "Where did you meet him?" "Was he good in bed?" He seriously asked me all that. I told him I didn't care for the guy (which is the truth) and that I didn't sleep with him or any of that. I really care about the friends with benefits guy to be honest, and he knows that but It's like... if he didn't stake his claim and ask me to be with him, I'm free to be with whoever and even though I want to be with him, I shouldn't have to wait around for him you know? lol